Patent GB-543748-A (GB)
Title: Improvements in or relating to a method of and apparatus for comparing alternating voltages
IPC: G01R1/28
Filed: 1940-09-09
Abstract: 543,748. Voltage and phase-difference measurements. ELLIOTT BROS. (LONDON), Ltd., HAWKES, H. D., and PETCH, H. S. Sept. 9, 1940, No. 14030. [Class 37] Two alternating voltages of the same frequency and between which there exists a known nominal relationship in respect of magnitude and phase, are compared by connecting the sources of the voltages in opposition through a differential winding on a magnetic coupling device, indicating any magnetic flux produced in the core of the coupling device, supplying to a standardizing winding a current of such magnitude and phase as to produce an additional flux to compensate for the nominal difference between the two voltages, supplying to a neutralizing winding a current to reduce the resultant flux to zero, and measuring the magnitude and phase of the neutralizing current. Fig. 1 shows a testing set for testing the ratio and phase angle errors of a potential transformer. On a soft iron ring 1 is wound a differential winding D connected in circuit with the secondaries of the transformer X under test and a standard transformer S so as to carry a current proportional to the difference in voltage and phase angle between the outputs of the two transformers. A standardizing winding ST is connected across the secondary of the standard transformer S when this has a different nominal value of secondary voltage from that of the transformer X and serves to standardize the flux in the ring 1 by compensating for the disparity in the nominal secondary voltage. The neutralizing winding N is in two main parts, the first part P carrying current in phase with the voltage of the standard transformer S, and the second part Q carrying current in quadrature therewith and comprising a main section Qm and a compensating section Qc carrying a correction current compensating for any in-phase error present, the two sections being connected so as to constitute a single winding carrying a pure quadrature current. The in-phase and quadrature parts P, Q of the neutralizing winding are connected through potentiometer resistances 2, 3 across the secondary of the standard transformer S. The fluxes of the windings ST, N oppose that due to the winding D, the neutralizing winding N being arranged so that its flux can be varied to reduce the resultant flux in the ring 1 to zero. The sliders of the resistances 2, 3 are calibrated to read direct in percentage ratio error and in minutes of phase angle error. A search coil G is connected to a vibration galvanometer VG for detecting the presence or absence of flux in the ring. In the circuit 4 including the opposed secondaries of the transformers S, X and the differential winding D is a three-position multicontact range-change switch 5. In the central position of the switch lever 5c a neon lamp 6 is connected in the circuit 4 and glows if the secondaries are connected in circulation instead of opposition. In the left and right hand positions X1, X5 resistances R9, R10 respectively are connected in series with the winding D to provide two ranges of measurement. The standardizing winding ST is connected through a multicontact current-reversing ratio-change switch 7 across the secondary of the standard transformer S through a high resistance R1. The switch 7 has three positions corresponding to three possible combinations of secondary voltages of the transformers and connects the appropriate tapping on a resistance 8 in series with the slide wires 2, 3 and also, in conjunction with the switch 5, connects resistances R3, R4, R5, R6, R7, R8 and the standardizing winding ST in different combinations across the secondary winding of the standard transformer S, so as to provide a constant burden on the transformer S for any combination of range measurement and ratio between the transformers S, X. When the voltages to be compared are nominally equal the standardising winding ST is on open circuit. Specification 527,143 is referred to.